MAX8729EEI+T Maxim Integrated Products, MAX8729EEI+T Datasheet - Page 22

• Manufacturer Part Number: MAX8729EEI+T
• Description: Display Drivers Constant-Frequency H alf-Bridge CCFL Inve
• Manufacturer: Maxim Integrated Products

Specifications of MAX8729EEI+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The MAX8729 senses the lamp current flowing through
resistor R1 (Figure 1) connected between the low-volt-
age terminal of the lamp and ground. The voltage
across R1 is fed to IFB and is internally full-wave recti-
fied. The MAX8729 controls the desired lamp current
by regulating the average of the rectified IFB voltage.
To set the RMS lamp current, determine R1 as follows:
where I
790mV is the typical value of the IFB regulation point
specified in the Electrical Characteristics table. To set
the RMS lamp current to 6mA, the value of R1 should
be 148Ω. The closest standard 1% resistors are 147Ω
and 150Ω. The precise shape of the lamp-current
waveform depends on lamp parasitics. The resulting
waveform is an imperfect sinusoid waveform, which has
an RMS value that is not easy to predict. A high-fre-
quency true RMS current meter (such as Yokogawa
2016) should be used to measure the RMS current and
make final adjustments to R1. Insert this meter between
the sense resistor and the lamp’s low-voltage terminal
to measure the actual RMS current.
The MAX8729 limits the transformer secondary voltage
during startup and lamp-out faults. The secondary volt-
age is sensed through the capacitive voltage-divider
formed by C3 and C4 (Figure 1). The voltage of VFB is
proportional to the CCFL voltage. The selection of par-
allel resonant capacitor C3 is described in the
Transformer Design and Resonant-Component
Selection section. Smaller values for C3 result in higher
efficiency due to lower circulating current. If C3 is too
small, the resonant operation is affected by the panel
parasitic capacitance. Therefore, C3 is usually chosen
to be between 10pF and 18pF. After the value of C3 is
set, select C4 based on the desired maximum RMS
secondary voltage V
where the 2.34V is the typical value of the VFB peak-
voltage when the lamp is open. To set the maximum
RMS secondary voltage to 1800V with C3 selected to
be 12pF, C4 must be less than or equal to 13nF.
Constant-Frequency, Half-Bridge CCFL
Inverter Controller
22
______________________________________________________________________________________
LAMP(RMS)
Setting the Secondary Voltage Limit
C
4
=
R
1
2
=
is the desired RMS lamp current and
x V
LAMP(RMS)_MAX
2 2
LAMP RMS MAX
Setting the Lamp Current
2 34
π
.
x I
x
(
LAMP RMS
790
V
mV
)_
(
:
)
x C
3
The MAX8729 limits the secondary current even if the
IFB sense resistor is shorted or transformer secondary
current finds its way to ground without passing through
R1. ISEC monitors the voltage across the sense resistor
R2 connected between the low-voltage terminal of the
transformer secondary winding and ground. Determine
the value of R2 using the following equation:
where I
transformer secondary current during fault conditions,
and 1.28V is the typical value of the ISEC peak voltage
when the secondary is shorted. To set the maximum
RMS secondary current in the circuit of Figure 1 to
22mA, set R2 = 40.2Ω.
The transformer is the most important component of the
resonant tank circuit. The first step in designing the
transformer is to determine the transformer turns ratio.
The ratio must be high enough to support the CCFL
operating voltage at the minimum supply voltage. The
transformer turns ratio N can be calculated as follows:
where V
normal operation, and V
voltage. If the maximum RMS lamp voltage in normal
operation is 800V and the minimum DC input voltage is
10V, the turns ratio should be greater than 178.
The next step in the procedure is to design the resonant
tank so the resonant frequency is close to the switching
frequency set by the HF resistor. The lamp current is clos-
er to sine wave when the switching frequency is close to
the resonant frequency The resonant frequency is deter-
mined by: the primary capacitive voltage-divider C
C
secondary leakage inductance L, and the CCFL lamp.
The simplified CCFL inverter circuit is shown in Figure 7a.
The half-bridge power stage is simplified and represent-
ed as a square-wave AC source. The resonant tank cir-
cuit can be further simplified to Figure 7b by removing the
transformer. C
tive divider reflected to the secondary and N is the trans-
former turns ratio.
S2,
the secondary parallel capacitor C
SEC(RMS)_MAX
LAMP(RMS)
Setting the Secondary Current Limit
Transformer Design and Resonant-
S
R
’ is the capacitance of the primary capaci-
2
=
N
is the maximum RMS lamp voltage in
≥
2
. 0 45
x I
V
IN(MIN)
is the desired maximum RMS
LAMP RMS
SEC RMS MAX
x V
1 28
.
Component Selection
(
(
IN MIN
is the minimum DC input
V
(
)_
)
)
P,
the transformer
S1
and